The goal of this research is to evaluate estrogen and isoflavone levels and action in premenopausal women after a defined consumption of isoflavone-rich soy products. Eighteen subjects will consume 3 diets for about 13.5 weeks each: their habitual diets (control diets), a low soy diet and a high soy diet. The soy product will be soy milk. A number of estrogen endpoints will be measured, reflecting estrogen availability, metabolism, excretion and action. Blood, urine, feces, food, exercise and menstrual diaries will be collected and endometrial biopsies will be performed.